In current 3GPP LTE (Third Generation Partnership Project Long Term Evolution) communication systems, that is, Releases 8, 9, and 10, downlink (DL) control signaling from an eNodeB is received by user equipment (UE) in the first, first two, or first three, or first four symbols of a subframe subsequently referred to as control symbols. The remaining symbols in the subframe, following the control symbols, are typically used for receiving user data. For example, FIG. 1 depicts an exemplary subframe structure of the prior art with three control symbols. Control signaling is spread across an entire carrier bandwidth (for example, 10 Megahertz (MHz)) of the first three symbols of the subframe and is received by the UE on a Physical Downlink Control Channel (PDCCH). User data is received by the UE on the Physical Downlink Shared Channel (PDSCH), and in select Resource Blocks (RBs) of the PDSCH occupying either in the entire carrier bandwidth or a portion of it.
In order to decode the information sent on PDCCH, the UE needs to perform channel estimation for coherent demodulation of the PDCCH. To perform channel estimation, the UE receives Reference Signals (RSs), for example, pilot symbols, that are Cell-specific reference signals (CRS) and included in the subframe and that are associated with one or more antenna ports. For example, in 3GPP LTE Releases 8, 9, and 10, the UE uses the CRSs associated with one or more of antenna ports 0, 1, 2, and 3 for receiving the PDCCH. The CRS structure for antenna ports 0, 1, 2, and 3 is shown in FIG. 1, wherein RSs labeled R0 are resource elements carrying RSs associated with antenna port 0, RSs labeled R1 are resource elements carrying RSs associated with antenna port 1, RSs labeled R2 are resource elements carrying RSs associated with antenna port 2, and RSs labeled R3 are resource elements carrying RSs associated with antenna port 3. An antenna port is defined such that a channel over which a symbol on the antenna port is conveyed can be inferred from the channel over which another symbol on the same antenna port is conveyed. Other signals and channels such as synchronization signals such as Primary/Secondary Synchronization channels (P/S-SCH), broadcast control channels, including primary broadcast control channel (PBCH), etc may also be present in a subframe. Typically, a master information block (MIB) is sent on the Physical Broadcast CHannel (PBCH), the MIB comprises of a portion of a system frame number (SFN), downlink system bandwidth and Physical Hybrid ARQ Channel (PHICH) configuration (such as duration and PHICH resource indicator). In LTE Release-8, the PBCH is sent on subframe 0 (each subframe comprising of two slots, each slot corresponding to a 0.5 milli-second) of a radio frame. The Synchronization signals are transmitted within the inner six PRBs or inner 72 subcarriers (approximately 1.1 MHz) of the carrier bandwidth on subframes 0 and 5 of radio frame. The exact location of the Synchronization signals changes based on the duplex type and Cyclic Prefix length.
For 3GPP LTE Release 10, in order to demodulate user data (sent on PDSCH), the UE can either use the RSs associated with antenna ports 0, 1, 2, and 3 or it can use RSs associated with other antenna ports, such as antenna ports 7, 8, 9, 10, 11, 12, 13, and 14, that is, the UE can use RSs associated with all or a subset of these antenna ports, based on the transmission scheme used for PDSCH reception (in turn, the transmission scheme depends on configuration signaling from a serving eNodeB). The RSs associated with antenna ports antenna ports 7, 8, 9, 10, 11, 12, 13, and 14 are typically referred to as “UE specific reference signals (UERSs)” or “Demodulation reference signals (DMRSs)” or “Dedicated reference signals (DRS).” The RSs associated with antenna ports 0, 1, 2, and 3 are typically referred to as “Common Reference Symbols (CRSs).” In transmission schemes based on CRS, the UE may use one or more of antenna ports 0, 1, 2, 3 and for transmission schemes based on DMRS, the UE may use antenna ports one or more of 7, 8, 9, 10, 11, 12, 13, 14. The actual number of spatial transmission layers and the associated antenna ports when using DMRS to decode PDSCH may be determined by the UE based on the downlink control channel (DCI) information associated with PDSCH. Typically, both CRS and DMRS are not simultaneously used to demodulate data in PDSCH. While the CRSs are sent across the entire carrier bandwidth by the eNodB, DMRSs can only be present in those RBs for which the UE has a PDSCH assignment. Therefore, when receiving PDSCH using DMRS, the UE can only use the DMRS present on those RBs for which it has a PDSCH assignment.
For 3GPP LTE Release 11 (the next generation LTE system), it is envisioned that new DL control signaling will be sent by the eNodeB to the UE in symbols that span a first time slot of the subframe or in symbols that span both the first and second time slots of the subframe. The new DL control signaling is generally referred to as Enhanced-PDCCH (EPDCCH). Unlike the PDCCH, which is transmitted across the entire channel bandwidth, a UE is expected to receive the EPDCCH in a set of RBs that may span only a portion of the carrier bandwidth in frequency domain. Also, unlike the PDCCH, which is received by the UE using CRS, it is envisioned that the EPDCCH can be received by the UE using DMRS
The new DL control signaling, that is, the EPDCCH, is expected to be used to complement the downlink control channels, that is, the PDCCH, of the existing 3GPP LTE Releases 8/9/10 for supporting features of Long Term Evolution-Advanced (LTE-A) Release 11+, such as CoMP (Coordinated Multi-point Transmissions) and enhanced Multiple-Input Multiple-Output (MIMO) techniques, including Multi-User MIMO (MU-MIMO). EPDCCH can allow advanced control channel transmission schemes such as beamformed frequency-selective control transmission, dedicated control transmission to a user via use of DMRS, spatially multiplexed control channel transmission to a single user MIMO (SU-MIMO) and MU-MIMO control transmission.
To receive EPDCCH, the UE has to perform blind decoding for several EPDCCH candidates, that is, EPDCCH signals that are possibly intended for the UE. In 3GPP LTE Release 10, in order to receive the PDCCH, the UE performs a maximum of 44 (60 if configured for uplink (UL) MIMO) blind decodes for the primary cell in each non-DRX (discontinuous reception) subframe. If the UE is configured for carrier aggregation (CA), the UE performs 32 additional blind decodes for each configured and activated secondary cell. In order to also receive the EPDCCH, the number of blind decodes that the UE has to perform increases significantly, imposing a significant time and processing load burden in the UE
Therefore, a need exists for mechanisms that ensure that the blind decoding complexity at the UE to receive both a PDCCH and an EPDCCH is kept at a reasonable level.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. Those skilled in the art will further recognize that references to specific implementation embodiments such as “circuitry” may equally be accomplished via replacement with software instruction executions either on general purpose computing apparatus (e.g., CPU) or specialized processing apparatus (e.g., DSP). It will also be understood that the terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.